1. Field of the invention
The invention relates to a non-recursive digital filter for generating binary output code words which occur at a given output sampling frequency f.sub.2 and are related in a predetermined manner to a sequence of binary input code words which occur at an input sampling frequency f.sub.1, the output sampling frequency f.sub.2 being an integral multiple (m) of the input sampling frequency f.sub.1. This digital filter comprises a storage device arranged to store a given number (N) of successive inpupt code words; a multiplying device for forming, within one input sampling period 1/f.sub.1, a number (m) of sets of products of the said given number (N) of input code words and a number (m) of different sets of weighting factors which correspond to the relationship between the input and the output code words; an adding device which is coupled to the said multiplying device and is adapted to generate, within one input sampling period 1/f.sub.1, a number of output codewords, which number corresponds to the number (m) of sets of products, each output code word being at least equal to the mathematical sum of all of the products of one particular set of products.
Such a digital filter arranged for increasing the sampling rate is known as an interpolating digital filter.
2. Description of the prior art
In a prior art interpolating non-recursive digital filter the storage device has the form of a delay line comprising a plurality (N) of delay sections each being arranged to store a single input code word. These delay sections are each connected to an adding device via a tapping which includes a multiplier. To each multiplier a weighting factor from a given set of weighting factors is supplied.
In particular, in the prior art digital filter, the input code words are subjected to a predetermined sequence of arithmetical operations to product an output code word. These operations can mathematically be represented by the convolution sum: ##EQU1## WHERE Q = 0, 1, 2, 3, . . . , M - 1.
In this expression:
m is an integer denoting the relationship between the input and output sampling frequencies (m = f.sub.2 /f.sub.1); PA0 N is an even number indicating the number of input code words stored in the input circuit; PA0 T is the reciprocal value of the input sampling frequency T = 1/f.sub.1 ; PA0 x[(n - k)T] is one input code word of the code words which occur with a period T = 1/f.sub.1 ; PA0 n is the ordinal number of the code word which occurs after the instant t = 0; PA0 y[(n + q/m)T] is one of the output code words which occur with a period T/m; PA0 C [q, k] is a weighting factor.
It should be noted that the weighting factors are commonly termed "filter coefficients" and are stored in a storage device such as, for example, a ROM.
However, to permit the above-described interpolating filter to perform the given mathematical operations it must have a size and a dissipation such that its practical usefulness is greatly restricted, for example with respect to the possibility of time multiplex processing of information signals.